1. Field
The present invention relates to a semiconductor device of a three-level power conversion circuit that is used for a three-level inverter or a resonant inverter.
2. Description of the Related Art
FIG. 4 shows an example of a circuit of a three-level three-phase inverter that converts a direct current (DC) into an alternating current (AC) according to the related art. DC power supplies 1 and 2 are connected in series in which a positive potential is P, a negative potential is N, and a neutral potential is M. In general, a DC power supply configured as an AC power supply system can be realized by using a structure in which a diode rectifier (not shown) is used to full-wave rectify the alternating current and a large-capacity electrolytic capacitor is used to smooth the rectified current.
Series-connected circuits corresponding to three phases, which are configured by connecting IGBTs (to which diodes are connected in reverse parallel in series), are connected between the positive potential P and the negative potential N. That is, a series-connected circuit 60 for a U phase is configured by connecting an upper arm including an IGBT 11 and a diode 12 connected in reverse parallel to the IGBT 11 and a lower arm including an IGBT 13 and a diode 14 connected in reverse parallel to the IGBT 13 in series. A series-connected circuit 61 for a V phase is configured by connecting an upper arm including an IGBT 21 and a diode 22 connected in reverse parallel to the IGBT 21 and a lower arm including an IGBT 23 and a diode 24 connected in reverse parallel to the IGBT 23 in series. A series-connected circuit 62 for a W phase is configured by connecting an upper arm including an IGBT 31 and a diode 32 connected in reverse parallel to the IGBT 31 and a lower arm including an IGBT 33 and a diode 34 connected in reverse parallel to the IGBT 33 in series.
An AC switch configured by connecting IGBTs (to which diodes are connected in reverse parallel) in series is connected between a series connection point between the upper arm and the lower arm of the series-connected circuit for each phase and a DC neutral potential M. That is, an AC switch circuit in which the emitter of a semiconductor device 63 (including an IGBT 81 and a diode 82 connected in reverse parallel to the IGBT 81) is connected to the emitter of a semiconductor device 64 (including an IGBT 83 and a diode 84 connected in reverse parallel to the IGBT 83), and devices 63 and 64 are connected between the series connection point of the series-connected circuit 60 for the U phase and the neutral point M of the DC power supply. In addition, an AC switch circuit in which the emitter of a semiconductor device 65 (including an IGBT 85 and a diode 86 connected in reverse parallel to the IGBT 85) is connected to the emitter of a semiconductor device 66 (including an IGBT 87 and a diode 88 connected in reverse parallel to the IGBT 87), and devices 65 and 66 are connected between the series connection point of the series-connected circuit 61 for the V phase and the neutral point M of the DC power supply. An AC switch circuit in which the emitter of a semiconductor device 67 (including an IGBT 89 and a diode 90 connected in reverse parallel to the IGBT 89) is connected to the emitter of a semiconductor device 68 (including an IGBT 91 and a diode 92 connected in reverse parallel to the IGBT 91), and devices 67 and 68 are connected between the series connection point of the series-connected circuit 62 for the W phase and the neutral point M of the DC power supply. The series connection points of the series-connected circuits 60, 61, and 62 are AC outputs of the U phase, the V phase, and the W phase and are connected to a load 74 through reactors 71, 72, and 73, each serving as a filter.
In the three-phase circuit structure, the series connection points of the series-connected circuits 60, 61, and 62 can output the positive potential P, the negative potential N, and the neutral potential M, respectively. Therefore, a three-level inverter output is obtained. The three-phase circuit structure is characterized in that it outputs an AC voltage with three voltage levels and fewer harmonic components, as compared to a two-level inverter. It is possible to reduce the sizes of the output filters 71 to 73.
Semiconductor devices in which the circuits corresponding to three phases shown in FIG. 4 are integrated into one module and semiconductor devices in which a circuit corresponding to one phase is integrated into one module have been manufactured. When a circuit corresponding to one phase is integrated into a module to form a semiconductor device, the semiconductor device can be used for a single phase. In addition, a plurality of semiconductor devices may be used to form the three-phase inverter shown in FIG. 4. FIGS. 5A and 5B show a semiconductor device including the circuit corresponding to one phase shown in FIG. 4. FIG. 5A shows the outward appearance of a semiconductor module and FIG. 5B shows the internal circuit structure. The semiconductor module includes as semiconductor elements IGBTs 11 and 13, diodes 12 and 14, and an AC switch 15. A terminal 17 is a C1 terminal that is connected to the positive potential P of the DC power supply. A terminal 18 is an M terminal that is connected to the neutral potential M of the DC power supply. A terminal 19 is an E2 terminal that is connected to the negative potential N of the DC power supply. A terminal 16 is an E1C2 terminal that is connected to a load. FIG. 5A shows a metal base substrate 3 allowing a semiconductor element or a wiring member to be provided thereon so as to be insulated, and an insulating case 4 of the module. The base substrate 3 also has a function of transferring heat generated from the inside of the module to a cooling fan. Any of the following substrates, for example, may be used as the base substrate 3: an aluminum insulating substrate having an insulating layer formed on an aluminum plate; and a substrate in which, for example, an alumina or aluminum nitride ceramic substrate having metal foil, such as copper foil, bonded thereto is mounted on a copper or alloy plate. In recent years, a ceramic substrate that has metal foil bonded thereto without a copper or alloy plate has been used as the base substrate 3. In all of the substrates, metal is exposed from the rear surface of the base substrate 3 and the semiconductor elements provided in the insulating case 4 are insulated from the metal by an insulator. In FIG. 5A, the terminals C1, M, and E2 are arranged in a line on the module. FIGS. 6A to 6C show examples of the structure of the AC switch 15 used in FIG. 5B. In the examples shown in FIGS. 6A and 6B, since a general IGBT has very low reverse blocking voltage capability, the IGBT and the diode are connected in series to ensure reverse voltage resistance. FIG. 6A shows the circuit structure of an AC switch formed by connecting the emitter of an IGBT 41 to which a diode 43 is connected in reverse parallel and the emitter of an IGBT 42 to which a diode 44 is connected in reverse parallel. When a current flows from a terminal K to a terminal L, the IGBT 41 is turned on and the current flows through a path from the IGBT 41 to the diode 44. When a current flows from the terminal L to the terminal K, the IGBT 42 is turned on and the current flows through a path from the IGBT 42 to the diode 43.
FIG. 6B shows the circuit structure of an AC switch formed by connecting the collector of the IGBT 41 to which the diode 43 is connected in reverse parallel and the collector of the IGBT 42 to which the diode 44 is connected in reverse parallel. When a current flows from the terminal K to the terminal L, the IGBT 42 is turned on and the current flows through a path from the diode 43 to the IGBT 42. When a current flows from the terminal L to the terminal K, the IGBT 41 is turned on and the current flows through a path from the diode 44 to the IGBT 41.
FIG. 6C shows the structure of an AC switch formed by connecting reverse-blocking IGBTs 45 and 46, which are IGBTs having reverse blocking voltage capability, in reverse parallel to each other. When a current flows from the terminal K to the terminal L, the reverse-blocking IGBT 45 turns on. When a current flows from the terminal L to the terminal K, the reverse-blocking IGBT 46 turns on (For example, see JP-A-2008-193779).
A circuit having the IGBTs connected in reverse parallel to each other or a circuit having the reverse-blocking IGBTs connected in reverse parallel to each other is given as an example of the AC switch. However, a combination of a diode bridge circuit and IGBTs or other kinds of semiconductor switching elements may be used.
The circuit structure shown in FIGS. 5A and 5B using the AC switch shown in FIG. 6A in which the emitters of the IGBTs are connected to each other as the AC switch 15 requires a total of four driving power supplies, that is, two driving power supplies for driving the IGBT 11 and the IGBT 13 and two driving power supplies for driving the IGBT 41 and the IGBT 42.
In the circuit structure shown in FIGS. 5A and 5B using the AC switch shown in FIG. 6B in which the collectors of the IGBTs are connected to each other as the AC switch 15, the emitter of the IGBT 11 is connected to the emitter of the IGBT 42 and the emitter potentials are equal to each other. Therefore, the IGBT 11 and the IGBT 42 can share a driving power supply, and the number of driving power supplies for driving the IGBTs can be reduced to three, that is, one driving power supply for driving the IGBT 11 and the IGBT 42 and two driving power supplies for driving the IGBT 13 and the IGBT 41. Since the number of driving power supplies is reduced, it is possible to reduce the size and cost of an inverter.
However, in the circuit structure shown in FIGS. 5A and 5B using the AC switch shown in FIG. 6B in which the collectors of the IGBTs are connected to each other as the AC switch 15, the number of driving power supplies is reduced, but the AC switch 15 has the following problems.
That is, in the case of a semiconductor device in which the circuits corresponding to three phases shown in FIG. 4 are integrated into one module or a semiconductor device in which a circuit corresponding to one phase is integrated into one module, the insulation test of such semiconductor is performed upon completion of the manufacture. In the insulation test, main terminals of the module and other terminals, such as control terminals, protruding to the outside of the module are connected to one terminal of an AC power supply, and metal exposed from the rear surface of the base substrate 3 is connected to the other terminal of the AC power supply. Then, for example, a voltage of 3.0 kV is applied to check electrical insulation between the semiconductor element in the module and the metal on the rear surface of the base substrate 3. The insulation test will be described with reference to FIGS. 7A and 7B. FIGS. 7A and 7B show an example in which a ceramic substrate 7 on a copper base 8 is used as a base substrate. As shown in FIG. 7A, when an AC power supply 9 supplies a current I and a positive voltage is applied to the terminal, charge is stored between a circuit pattern (not shown) on the ceramic substrate 7 (represented by a dotted line) and the copper base 8 on the rear surface of the ceramic substrate 7. In this case, the following three charges are stored between the circuit pattern of the ceramic substrate 7 and the copper base 8 in the AC switch: a charge Q1 that is stored by a charging current I11, part of a current I1, in a capacitive component C1 between the emitter of the IGBT 41 and the copper base; a charge Q2 that is stored by a charging current I21, part of a current I2, in a capacitive component C2 between the emitter of the IGBT 42 and the copper base; and a charge Q3 that is stored in a capacitive component C3 between the cathode and the copper base by a charging current I3+I4 obtained by a current I3 flowing across the diode 43 and a current I4 flowing across the diode 44.
Then, as shown in FIG. 7B, when the voltage applied from the AC power supply 9 is reduced, the charge stored in the ceramic substrate 7 is discharged. At that time, the discharging currents I11 and I21 that are generated by the charges Q1 and Q2 stored in the capacitive components C1 and C2 between the emitters of the IGBTs 41 and 42 and the copper base, flow to the AC power supply. However, the discharging current I3+I4 that is generated by the charge Q3 stored in the capacitive component C3 between the collector and the copper base, is prevented from flowing by the diode and remains without being discharged. Therefore, there is a large potential difference between the collector and the emitter of the IGBTs 41 and 42 due to the charge Q3 remaining in the capacitive element C3 between the collector and the copper base. Therefore the IGBTs 41 and 42 are likely to be damaged.
In the AC switch shown in FIGS. 7A and 7B, since the collector is shared between the IGBT 41 and the IGBT 42, auxiliary emitters 6c and 6d are provided at both ends of the AC switch. Therefore, when the auxiliary emitters 6c and 6d are also used to evaluate an individual element, the overall characteristics of the IGBTs and the diodes, such as the IGBT 41 and the diode 44, and the IGBT 42 and the diode 43, are measured. Therefore, it is difficult to evaluate the individual element.